1. Field of the Invention
The present invention relates, generally, to the field of radio frequency signal generation and wireless communications, and particularly to low power portable wireless communications, examples of which include cellular radio and Personal Communications Systems (PCS), point to point data communications, wireless data acquisition, and wireless automatic identification, remote control, and status reporting.
2. Description of the Related Art
The radio link performance for "MicroRadio.TM." equipment such as wireless data acquisition, local area data links, garage door openers, vehicle toll tags, RF auto ID, and remote keyless entry systems is often limited by the high emphasis placed upon small size and low cost. In recent years, designers of portable communications equipment such as cellular and PCS handsets have also been under severe pressure to reduce size, cost, and power consumption. Progress in equipment design in these fields requires advances in performance where the necessary low cost and size are still maintained. MicroRadio is a trademark of Cleveland Medical Devices Inc.
Early radio equipment used frequency control based on free running (not contained in a control loop) inductor/capacitor (LC) resonator oscillators. The equipment was plagued with frequency stability problems, and had no facility for anything other than manual tuning. With the advent of software controlled phase locked loop (PLL) frequency synthesizers, a stable, frequency agile, software controlled RF source became available. Advances in electronic technology, and particularly in integrated circuit design, has resulted in size reduction to the point of high portability. These compact synthesizers allow transmitters and channel defining local oscillators in receivers to be set via software control to any of a large number of frequencies uniformly distributed across the band of interest. The most recent advances make feasible the application of frequency synthesis to MicroRadio systems provided that modulation can be provided for without unduly increasing the size and power consumption.
There is a long history of attempts to directly modulate PLL synthesizers, with varying degrees of efficiency and success. The structure and operation of a PLL makes high performance efficient modulation appear quite feasible. The PLL has within it an Electronically Controlled Oscillator (ECO), which typically uses voltage as the frequency control variable and is thus usually referred to as a Voltage Controlled Oscillator (VCO). Although for the sake of convenience "voltage" is often referred to as the signal or control variable, it is understood that current or another electronic variable could be substituted. The generation of the precise voltage (or other electronic variable as the case may be) needed to steer the VCO to the desired frequency is the main function of the system. Simply summing a modulating voltage into the resulting steering voltage will frequency modulate the PLL around the carrier frequency, but results in serious difficulties. The primary problem revolves around the fact that the PLL views the modulation as error that it must correct, resulting in distortion of the desired modulation. A long patent history testifies to the work that has been invested in attempting to effectively solve this elusive problem, the net result of which has been only a partial success.
U.S. Pat. No. 3,393,380 granted to Webb and assigned to the National Aeronautics and Space Administration, PHASE LOCKED PHASE MODULATOR INCLUDING A VOLTAGE CONTROLLED OSCILLATOR, 1968, presents the earliest example of frequency modulation of a phase locked loop. This simple early form injects the modulation into the phase detector output, which has the disadvantages of limited modulation rate due to the filtering action of the subsequent low pass loop filter, as well as allowing distortion to be introduced by the response of the phase locked loop to the modulation. It did, however, represent an advance on the frequency modulation techniques used at the time. The intended application was space communications. No method of preventing the phase locked loop from distorting the desired FM is presented.
A fundamental method of improving the modulation capability of a PLL is to modulate its reference frequency in sympathy with its electronically controlled oscillator. The goal of this is to prevent the loop phase detector from detecting the modulation as frequency error, and leading to the PLL distorting the modulation through its closed loop action of driving all frequency error to zero. There does not appear to be any existing patent on this method, but it has been known in the art for many years. For example, it is implied in U.S. Pat. No. 4,052,672 granted to Enderby et al. in 1977, and described in detail without reference to any patents as well known prior art by Drucker in U.S. Pat. No. 4,313,209, filed in 1980 and issued in 1982. The method is limited by the low modulation bandwidth of practical voltage controlled crystal oscillators (VCXOs) normally used as reference frequency sources. Again, it is noted that a frequency steerable reference frequency source that is crystal based is in more general terms described as an Electronically Controlled Crystal Oscillator (ECXO), and that in the most general terms where the reference source is not necessarily crystal based it may be referred to as an Electronically Controlled Reference Source (ECRS). Unmentioned in previous patents, but shown in detail herein, is that for low distortion modulation the normally crystal based reference oscillator modulation bandwidth must greatly exceed the PLL loop bandwidth. Since the modulation bandwidth of crystal oscillators is very limited, the PLL loop bandwidth becomes extremely limited. This results in long lock times and elimination of the benefits of wider loop bandwidths, such as suppression of vibration induced noise. The nonlinearity of the common VCXO is a source of large distortion in this method, particularly for wider frequency deviations. However, practical linearizing techniques to deal with the high nonlinearity of typical VCXO's are not commonly known in the art.
U.S. Pat. No. 4,052,672, EXTENDED PHASE-RANGE, HIGH FIDELITY MODULATION ARRANGEMENT issued to Enderby in 1977 and assigned to Motorola, Inc., presents another fundamental improvement to the modulated PLL that, like the modulated reference method, has the purpose of eliminating phase detector output that would normally result from modulation. A "correcting integrator" is added to the system that generates a voltage of equal magnitude and opposite sign to the phase detector output component due to modulation applied to the VCO. This correcting integrator output is summed with the phase detector output to cancel loop response to modulation. This breakthrough concept is quite elegant in the way it attacks the root of the problem, and subsequently evolved into the major technique applied for frequency modulating the phase locked loop synthesizer. Numerous subsequent patents have incorporated this excellent basic technique. The method shall be referred to herein as the "two point correcting integrator form", as modulation is applied to the correcting integrator and the voltage controlled oscillator, or as the "Enderby form". While the Enderby form is effective and highly circuit efficient, it does not allow DC modulation due to the fact that build up of phase error associated with DC modulation will eventually cause either the correcting integrator or the phase detector to exceed their dynamic ranges, at which point the PLL will either lose lock or lose the effect of the correcting integrator. Also not mentioned is the fact that with digital phase detectors the encoding of phase error as pulse width introduces noise and distortion that may set severe limits on the low frequency content of the modulation or on the allowed phase locked loop bandwidth.
The Enderby two point correcting integrator form is incorporated and extended in numerous subsequent patents. This refinement process begins with U.S. Pat. No. 4,242,649, METHOD AND APPARATUS FOR MODULATING A PHASE LOCKED LOOP, issued to Washburn in 1980 and assigned to Harris Corporation. The refinement is the addition of modification of the correcting integrator transfer function to take into account second order effects in the PLL. These are significant improvements for practical design, but do not address the lower frequency limits of the method.
U.S. Pat. No. 4,313,209, PHASE-LOCKED LOOP FREQUENCY SYNTHESIZER INCLUDING COMPENSATED PHASE AND FREQUENCY MODULATION, issued to Drucker in 1982 and assigned to John Fluke Mfg. Co., Inc., extends Enderby to also include phase modulation. For the phase modulated form the correcting integrator becomes a correcting amplifier, and the modulating signal summed into the VCO input is differentiated to compensate for the fact that the VCO act as integrator of input voltage to output phase. This invention also incorporates a mixer and a second phase locked loop embedded within the main loop in order to provide means for finer frequency stepping and for filtering any mixer spurious products. The method does not provide for DC frequency modulation capability.
U.S. Pat. No. 4,546,331, FREQUENCY MODULATION IN A PHASE LOCKED LOOP, issued to DaSilva in 1985 and assigned to Hewlett Packard Company, solves the problems of limited correcting integrator and phase detector dynamic ranges, the first extension of Enderby allowing DC modulation of the two point correcting integrator form. The solution involves a complex method of removing charge from the correcting integrator and a corresponding amount of phase error from the phase detector. The method is highly functional, but requires custom design of PLL subsystems that does not take advantage of available phase locked loop synthesizer integrated circuits. Its complexity and power consumption make it suitable for larger equipment. U.S. Pat. No. 4,573,026, FM MODULATOR PHASE LOCKED LOOP WITH FM CALIBRATION, issued to Curtis in 1986 and assigned to Hewlett Packard company, presents a method of digitally "linearizing" a voltage controlled oscillator with respect to a control voltage. This method is intended to be utilized in the design of frequency modulated phase locked loops. Like the DaSilva patent, the method involves custom high performance circuitry for precisely adding and remove charge corresponding to two pi radians of phase shift. The patent illustrates the need for compensation of unit to unit and frequency variable parameters in the subsystems of a modulated PLL in order to maintain performance.
U.S. Pat. No. 4,866,404, PHASE LOCKED FREQUENCY SYNTHESIZER WITH SINGLE INPUT WIDEBAND MODULATION SYSTEM, issued to Vandegraaf in 1989 and assigned to General Electric Company, is a variation on the idea of the basic correcting integrator patent of Enderby. In this variation the bypassed correcting integrator is not summed into the phase detector output. Instead additional electronic functions equivalent to scaling and the loop filter are cascaded with the correcting integrator and the resulting output summed into the VCO control input or inputs in order to cancel loop response to modulation as phase error. Another way of saying this is that the summing in of the correcting integrator occurs on the output side of the loop filter instead of the input side, thus requiring a copy of the loop filter to be inserted in order to arrive at the same transfer function. This method offers no fundamental performance advance over the originally presented Enderby form, has the same weakness of not allowing DC modulation, and has the disadvantage of additional circuitry.
U.S. Pat. No. 5,097,230 issued to Lautzenhiser in 1992 and assigned to Emhiser Research Limited, PHASE LOCKED LOOP THAT INCLUDES DC MODULATION, presents another method that allows the highly desirable feature of DC modulation in a phase locked loop. The method involves a mixer in the feedback loop that allows a second modulated signal source to be mixed with the desired VCO output so that the mixer output may remain constant under modulation. This complicated technique and other similar methods are highly applicable and useful in classes of equipment that are not extremely size, cost, and power limited. However, like the DaSilva and Curtis patents, its complexity make it suitable for non-portable wireless applications.
U.S. Pat. No. 4,670,888, issued to Smith and assigned to Agile Systems, details a method of modulation based on reprogramming the main voltage controlled oscillator divider in sympathy with digital data. This method is limited to frequency deviations equal to multiples of the channel stepping frequency of the synthesizer, and suffers from unacceptable distortion in practice unless the loop bandwidth is made extremely low. The need for high divider programming update rates requires high speed parallel divider programming, thus leading to physically larger designs than the more common and convenient serially loaded synthesizer integrated circuits would allow.
U.S. Pat. No. 5,266,907, issued to Dacus and assigned to Timeback FLL, details a new method of modulatable frequency synthesis based on a frequency locked loop using a delta sigma frequency detector. The method eliminates many of the difficulties involved in modulation of a phase locked control loop, due to the fact that loop response to frequency modulation may be cancelled with only a correcting amplifier as opposed to an integrator. However, it is not conveniently implemented with available components due to the lack of commercially manufactured synthesizer integrated circuits based on the new concept of a delta sigma frequency encoder.
With regards to the problem of initiating digital modulation without introducing frequency error, U.S. Pat. No. 5,493,257 to Chadwick and assigned to Plessy Semiconductor Limited, MODULATOR WITH BIASING CIRCUIT TO MINIMIZE OUTPUT DISTORTION, presents a method for efficient implementation of "midpoint" modulation in an FM PLL system. Midpoint modulation may be defined as a method of initiating modulation where frequency is initially at the nominal carrier frequency and deviates out to the state that defines a logic one or zero, with the initial deviation one half of the normal peak to peak deviation. If the very first deviation were equal to the nominal peak to peak, starting from the midpoint would deviate the first swing too far (twice too much), and would result in an undesired transient response while the PLL brings the midpoint of the frequency deviation back to the nominal carrier frequency. The technique presented by Chadwick performs proper midpoint modulation by AC coupling of the desired modulation, so it prevents excess deviation and resulting transient response at the expense of a lower limit on the modulation or data rate. U.S. Pat. No. 4,609,886 to Takaki and assigned to Pioneer Electronic Corporation, PLL MODULATION CIRCUIT, presents a method of attaining true DC coupling with midpoint modulation. The present invention includes significantly simpler methods of attaining similar results.
None of the above patents teach any methods that provide for the combination of high quality frequency modulation with DC modulation capability, convenient calibration for unit to unit, temperature, and frequency variation, and that are suitable to highly portable low power equipment manufactured with commercially available components. The modulation initiation solutions presented either ignore DC modulation or are overly complex for this class of equipment as well. Accordingly, a need still exists for an apparatus which provides high quality DC and AC modulation of a phase locked loop synthesizer within the size, cost, and power constraints of MicroRadio and other highly compact portable wireless applications.